Some motorcycles are equipped with a throttle body constructed in such a manner that two valves, namely, a main throttle valve and a sub-throttle valve are provided in an air-intake device for controlling an amount of air taken into an engine from outside such that they are arranged in series in an air flow direction. The main throttle valve is opened and closed in association with a rider's throttle grip operation whereas the sub-throttle valve is driven by a motor and its opening degree is controlled by an ECU (electronic control unit). When the main throttle valve is abruptly opened or closed by the rider's abrupt throttle operation, the opening degree of the sub-throttle valve is controlled so as to reduce a rapid change in the amount of air taken into an engine to thereby change the engine speed smoothly. This provides the rider with a comfortable feeling ride.
In some throttle bodies, upon the sub-throttle valve being opened with the main throttle valve closed, rotation of the sub-throttle valve is transmitted to the main throttle valve through a link system (see Japanese Laid-Open Patent Application Publication No. 2005-106048). To be specific, during cold start (first idling), the ECU opens the sub-throttle valve for a predetermined time period and thereby opens the main throttle valve by a small angle to obtain a desired amount of air, enabling stable combustion.
In a motorcycle with such a throttle body, when a throttle grip is returned to cause the engine to decelerate from a high-engine speed condition and a clutch is turned off, the main throttle valve is closed, causing the amount of air to rapidly decrease. And, the engine runs idle under the condition in which a rotational inertia force is not transmitted from a drive wheel to the engine. As a result, the engine speed tends to decrease. In particular, since weight of a fly wheel mounted to a crankshaft of the engine is small in the motorcycle, the engine speed significantly decreases during deceleration associated with the off-operation of the clutch as compared to automobile. If the engine speed decreases excessively during the deceleration in the clutch off-state, then smooth operation of the engine is difficult to achieve, and the rider is unable to obtain a comfortable feeling ride when the engine is accelerating after the deceleration. In addition, combustion tends to be unstable. As a result, unfavorable emission of exhaust gas occurs.